One of the main drivers for package design rules is the input/output (I/O) density per mm per layer (IO/mm/layer). The I/O density may be limited by the via pad sizes. However, current packaging technologies limit the extent to which the size of the via pads may be reduced.
Traditionally organic substrate manufacturing is performed utilizing semi-additive processing (SAP), with interconnections between layers made by laser drilling processes. Such interconnections include at least one vertical interconnect access (via) that includes a pad. Currently, via pads need to be relatively large due to the laser drilling processes used to create via openings through a dielectric layer above the via pads. Laser drilling is limited by the minimum feature size and the misalignment of the laser when drilling via openings. Some lasers, such as UV lasers, can reduce the via opening more than other types of lasers, but throughput is also greatly decreased.
As explained above, current laser drilling processes may result in creation of an alignment margin that requires a pad beneath a via that is larger than an opening of the via (via opening). This relatively large pad (when compared to the via opening) may limit the I/O density of a device, which may exacerbate difficulties associated with achieving I/O densities that are equal to or greater than 50 IO/mm/layer. One alternative to the laser drilling processes described above is a process of fabricating a zero-misalignment via (ZMV). The process of creating a ZMV (ZMV process) can be used to fabricate vias and pads that can increase I/O densities (when compared to the I/O densities achieved by laser drilling processes). The ZMV process method utilizes a photoresist layer with sensitivity to two different light wavelengths, two different light intensities, two different regions of gray-scale photomask, or a combination thereof. In this way, the photoresist layer can be differentially patterned in conjunction with a dose sensitive resist layer. This allows the vias and traces to be plated in a two-step process without removal of the photoresist layer. Consequently, the ZMV process can assist with avoiding any alignment impact on these layers. In the ZMV process, the line width and line spacing—that is, the pitch—is limited by the resolution of the exposure tool and the resist capability. Another approach of the ZMV process includes use of a dual color—i.e., a dual tone resist that is sensitive to two distinct wavelengths. There are, however, some drawbacks to these approaches. Special resist materials, such as liquid resists, are required for a ZMV process that uses a dual tone resist. In addition, utilizing the previously discussed methods results in a via shape that is not well defined in the direction along the trace and this may have an effect on via's reliability.